Chemical Decontamination Method and Chemical Decontamination Apparatus

Abstract

Provided is a chemical decontamination method that shortens the decomposition time of a reduction decontamination agent. An oxidization decontamination, a decomposition of an oxidization decontamination agent, and reduction decontamination using an oxalic acid aqueous solution are performed on a target piping of a BWR plant. After that, the oxalic acid is decomposed (S 7 ). That is, a part of the oxalic acid is decomposed by irradiating the oxalic acid aqueous solution with ultraviolet rays upstream of a decomposition device (S 8 ), and Fe 3+ in the aqueous solution is converted to Fe 2+ . Hydrogen peroxide is supplied to the decomposition device (S 9 ). In the decomposition device, the oxalic acid is decomposed by a catalyst and hydrogen peroxide, Fe 2+ and hydrogen peroxide react to produce Fe 3+ and OH*, and the oxalic acid is decomposed by OH*. A corrosion potential of the aqueous solution flowing out from the decomposition device is measured (S 11 ). A concentration ratio calculation device obtains Fe 3+ /Fe 2+ (concentration ratio) based on the corrosion potential (S 12 ), and A control device controls the supply amount of hydrogen peroxide to the decomposition device based on Fe 3+ /Fe 2+ (S 14 and S 16 ).).

1 . A chemical decontamination method comprising a step of performing a reduction decontamination of a constituent member of a nuclear power plant by bringing an aqueous solution of a reduction decontamination agent into contact with a surface of the constituent member which is to be in contact with a reactor water, and thereby decomposing the reduction decontamination agent contained in the aqueous solution,
 wherein said step includes the steps of:   measuring a corrosion potential of the aqueous solution discharged from a decomposition device to which an oxidizing agent is supplied;   determining a concentration ratio of Fe 3+  to Fe 2+  of the aqueous solution based on the measured corrosion potential; and   controlling a supply amount of the oxidizing agent to the decomposition device based on the concentration ratio.   
     
     
         2 . The chemical decontamination method according to  claim 1 ,
 wherein the determined concentration ratio is displayed.   
     
     
         3 . The chemical decontamination method according to  claim 1 ,
 wherein the aqueous solution before being supplied to the decomposition device is irradiated with ultraviolet rays.   
     
     
         4 . The chemical decontamination method according to  claim 1 ,
 wherein the reduction decontamination agent contained in the aqueous solution is decomposed by an ultraviolet rays irradiation to the aqueous solution, and the reduction decontamination agent is decomposed by a catalyst present in the decomposition device and the oxidizing agent supplied to the decomposition device.   
     
     
         5 . The chemical decontamination method according to  claim 1 ,
 wherein it is determined whether the supply amount of the oxidizing agent to the decomposition device is excessive based on the concentration ratio.   
     
     
         6 . The chemical decontamination method according to  claim 5 ,
 wherein the supply amount of the oxidizing agent to the decomposition device is determined to be excessive when the concentration ratio is greater than 1.   
     
     
         7 . The chemical decontamination method according to  claim 5 ,
 wherein the supply amount of the oxidizing agent to the decomposition device is decreased when the supply amount of the oxidizing agent to the decomposition device is excessive.   
     
     
         8 . The chemical decontamination method according to  claim 5 ,
 wherein the supply amount of the oxidizing agent to the decomposition device is increased so that the concentration ratio becomes a concentration ratio set value when it is determined that the supply amount of the oxidizing agent to the decomposition device is not excessive and it is determined that the concentration ratio is smaller than the concentration ratio set value which is 1 or less.   
     
     
         9 . The chemical decontamination method according to  claim 1 ,
 wherein the aqueous solution discharged from the decomposition device is stirred upstream of a position at which the corrosion potential is measured.   
     
     
         10 . The chemical decontamination method according to  claim 1 ,
 wherein the control of the supply amount of the oxidizing agent to the decomposition device based on the concentration ratio is that the concentration of the oxidizing agent in the aqueous solution flowing into the decomposition device is determined based on the concentration ratio, and the supply amount of the oxidizing agent to the decomposition device is controlled based on the concentration of the oxidizing agent.   
     
     
         11 . A chemical decontamination method comprising the steps of:
 forming a closed loop including a first piping and a second piping by connecting the second piping with a first piping to be chemically decontaminated, the first piping being communicated with the reactor pressure vessel and being a constituent member of a nuclear power plant, the second piping being different from the first piping; and   performing a reduction decontamination on an inner surface of the second piping by supplying an aqueous solution containing a reduction decontamination agent from the second piping to the first piping,   in a step of decomposing the reduction decontamination agent contained in the aqueous solution,   measuring a corrosion potential of the aqueous solution connected to the second piping and returned from the first piping to the second piping, and the aqueous solution discharged from the decomposition device to which the oxidizing agent is supplied;   calculating a concentration ratio of Fe 3+  to Fe 2+  in the aqueous solution based on the measured corrosion potential; and   controlling the supply amount of the oxidizing agent to the decomposition device based on the concentration ratio.   
     
     
         12 . The chemical decontamination method according to  claim 11 ,
 wherein the aqueous solution before being supplied to the decomposition device by the second piping is irradiated with ultraviolet rays.   
     
     
         13 . The chemical decontamination method according to  claim 12 ,
 wherein the reduction decontamination agent contained in the aqueous solution is decomposed by the ultraviolet rays irradiation to the aqueous solution, and   the reduction decontamination agent is decomposed by a catalyst present in the decomposition device and the oxidizing agent supplied to the decomposition device.   
     
     
         14 . A chemical decontamination apparatus comprising:
 a circulation piping which is connected to a piping system of a chemical decontamination target which is a constituent member of a nuclear power plant and supplies an aqueous solution containing a reduction decontamination agent to the piping system;   a corrosion potential measuring device for measuring the corrosion potential of the aqueous solution connected to the circulation piping and discharged from the aqueous solution and the decomposition device oxidizing agent is supplied in the circulation piping;   a concentration ratio calculation device for determining a concentration ratio of Fe 3+  to Fe 2+  of the aqueous solution based on the corrosion potential measured by the corrosion potential measuring device; and   a control device that controls a supply amount of the oxidizing agent to the decomposition device based on the concentration ratio determined by the concentration ratio calculation device.   
     
     
         15 . The chemical decontamination apparatus according to  claim 14 , comprising an ultraviolet rays irradiation device which irradiates the aqueous solution supplied to the decomposition device with ultraviolet rays. 
     
     
         16 . The chemical decontamination apparatus according to  claim 14 , comprising a mixing device which stirs the aqueous solution discharged from the decomposition device upstream of a position at which the corrosion potential is measured.